Frequency-domain (or “swept-source”) optical coherence tomography (OCT) systems are powerful tools that provide non-invasive, high-resolution images of biological samples at higher acquisition speeds and lower signal-to-noise ratios than time-domain OCT systems. FIG. 1 illustrates an exemplary frequency-domain OCT system 100 at a high level. As shown, the exemplary OCT system includes a wavelength-swept laser source 95 (also referred to herein as a frequency swept source) that provides a laser output spectrum composed of single or multiple longitudinal modes to an input of a coupler 72. The coupler 72 divides the signal fed thereto into the reference arm 80 that terminates in the reference mirror 82 and the sample arm 84 that terminates in the sample 86. The optical signals reflect from the reference mirror 82 and the sample 86 to provide, via the coupler 72, a spectrum of signals that are detected by a photodetector 88.
FIG. 2 is a plot 200 graphically illustrating a detection and ranging-depth arrangement for a typical frequency-domain OCT system, such as, for example, the system 100 of FIG. 1. As depicted, an envelope of coherence function 210 (or “fringe visibility curve”) defined by the instantaneous output spectrum of the swept source and the detection frequency of the system is plotted in the frequency domain. In this example, both the source output spectrum and the fringe visibility curve are Gaussian. As will be appreciated, the positive and negative frequency bands are not differentiable in the electrical domain. Accordingly, the images associated with the positive and negative frequency bands, respectively, are overlapped. As a result of this ambiguity, only half of the frequency range, corresponding to positive depth, is used for the imaging range 220. The upper frequency bound of the imaging range 220 is typically matched to the 6 dB roll-off (Zc) of the coherence function 210, which is referred to as the ranging depth.
The limitation on ranging depth (or the imaging depth range) illustrated in FIG. 2 has been ameliorated to some extent via the incorporation of the carrier-frequency heterodyne detection scheme described in U.S. Pat. No. 7,733,497 (the '497 patent), the entire disclosure of which is incorporated by reference herein. FIG. 3 is a plot 300 graphically illustrating a shifting of the frequency band by a carrier frequency (fs) 301 in accordance with the scheme of the '497 patent. As depicted, an envelope of coherence function 310 is shown. The function 310 is defined by the instantaneous output spectrum of the swept source and the detection frequency band, wherein the frequency band is shifted by the carrier frequency 301. As will be appreciated, this shifting doubles the ranging depth 320, as both sides of the frequency band centered at the carrier frequency (fs) 301 produce images without ambiguity. Additionally, as shown, the Nyquist frequency (fnyquist) 303 is typically double the carrier frequency (fs) 301. Furthermore, the Nyquist frequency, as known in the art, may be one-half of the sampling rate of the system.
However, artifacts that result in sub-optimal imaging may in some circumstances, plague even such carrier-frequency heterodyne detection schemes. For example, FIG. 4, shows the plot 300 and further illustrates foldover artifacts 331 (i.e., aliasing artifacts). The artifacts 331 may result from portions of the coherence function 310 beyond the DC (i.e., frequency=0) limit and/or the Nyquist frequency limit. Furthermore, the artifacts 331 may manifest even when the frequency band has been shifted by a particular carrier frequency (fs) 301. Such artifacts may be caused by reflections from structures outside of the imaging range of the system (due to, e.g., non-optimal sample placement) and may lead to aberrations in the OCT images. Thus, there is a need for frequency-domain OCT systems and techniques that eliminate such aliasing artifacts while enabling larger imaging ranges.